Water: Water Quality Standards Academy
Basic Course: Supplemental Topics - Health (x)
- Listing Impaired Waters and Developing TMDLs
- Monitoring & Assessment
- NPDES Permit Program
- Human Health Ambient Water Quality Criteria
- Historical Approach to Human Health Criteria Development
- Updated Approach
- Quantitative Risk Assessment
- Toxicological Parameter for Noncancer Effects
- Risk Assessment for Noncancer Effects
- Toxicological Parameter for Cancer Effects (Linear)
- Risk Assessment for Cancer Effects (Linear)
- Toxicological Parameter for Cancer Effects (Nonlinear)
- Risk Assessment for Cancer Effects (Nonlinear)
- Exposure Assessment
- Aquatic Life Criteria
Exposure Assessment: RSC
A complete human exposure assessment goes beyond estimates of exposure to the toxic pollutant of concern from ambient water and consumption of fish/shellfish. It also addresses exposure by way of other routes, including:
- Inhalation from airborne sources.
- Dietary intake (not including from fish/shellfish).
- Consumption of marine aquatic organisms.
- Recreational contact.
An analysis of overall exposure based on available data and the contributions from each source is called the Relative Source Contribution (RSC) analysis. A data-based RSC is determined when data are available. When data are not available, a default assumption is used. It is important to look for data before using a default value. The primary media to consider are foods (other than the fish in the equation), ambient air, and drinking water.
An RSC analysis is not used when the toxic pollutants of interest have cancer effects that are linear (i.e., those associated with a risk level). For those with nonlinear cancer effects, the RSC parameter is expressed as a percentage or subtracted from the RfD, depending on the circumstances.
Resource. The 2000 Human Health Methodology provides a decision-tree approach to determining the RSC. The approach is beyond the scope of this module. However, it is described in Chapter 4 of the EPA Methodology for Deriving Ambient Water Quality Criteria for the Protection of Human Health (October 2000).
Key Point. Independent of the process used in determining the RSC, there is a 20% floor and an 80% ceiling to account for the difficulty in identifying all possible routes of exposure.